In most land-based applications, navigation is often aided by in-place infrastructure such as GPS, radio beacons or a priori maps. Navigation and mapping underwater is difficult because among other things, wide-coverage underwater GP-equivalents do not exist and large portions of the sea bed are still unexplored.
Current techniques for underwater navigation use publicly available bathymetry maps. However, these maps are relatively coarse and unsuitable for precision navigation. Other sonar-based navigation systems rely on positioning schemes that use the sonar data itself. For example, on-the-fly acoustic feature-based systems attempt to use sonar to detect naturally occurring landmarks. Other solutions to the navigation problem include deploying low-cost transponders in unknown locations thereby enabling range-based measurements between the vehicle and transponder beacon. However, these transponders are often deployed at locations that are at great distances from each other, and often only partially observable because of the range-only information. Thus, these technologies are unsuitable for navigation across small vehicle paths.
Recent technologies permit navigation of underwater terrain relative to a prior map of the terrain. Such technologies use synthetic aperture sonar systems for generating images of the terrain, which are then compared against a prior image associated with the terrain. Underwater vehicles may then be able to navigate on the terrain relative to their location on the map. These technologies, however, suffer from a plurality of deficiencies including the amount of power consumed, size and shape of the systems. Additionally, the performance of such navigation systems dramatically decreases as transmitter frequencies increase and wavelengths decrease.
Accordingly, there is a need for improved map-based navigation systems, particularly for underwater applications.